1. Field of the Invention
The present invention relates to a reflector filter operating with surface acoustic waves, having at least one input transducer, at least one output transducer and at least one reflector structure. The reflector structure(s) acts (act) as a 180.degree. reflector, and the input transducer, output transducer and reflector structure are arranged in-line on the surface acoustic wave substrate.
The U.S. Pat. Nos. 4,484,160 and 4,520,330 disclose a surface acoustic wave reflector filter, known as a RAC filter, having an input transducer, an output transducer and two reflector structures which, with reflector fingers positioned obliquely, face each other (lying next to one another) in such a way that an acoustic wave proceeding into one reflector is deflected by approximately 90.degree. and proceeds into the other reflector. The acoustic wave is deflected by this second transducer with an approximately 90.degree. change of direction to the output transducer. A filter of this type is suitable for use to define a time window, or time span for monitoring events, for example. The finger structures of the aforesaid reflectors can be weighted and/or dispersive in accordance with the desired transmission function. In particular, finger displacement weighting, finger omission weighting and finger rotation can be provided as finger weighting. A substrate with a reduced length is required for such a surface acoustic wave reflector filter since, despite the considerable length of the reflectors, no substrate length corresponding to a conventional in-line arrangement is required as a result of the folded acoustic path.
A disadvantage of a reflector filter of this type is that there is a certain temperature dependency of the angle accuracy of the 90.degree. reflection, even in the case of a substrate made of quartz. A filter of this type also has a certain amount of transmission losses.
It is important for such a surface acoustic wave reflector filter that the transmission of the electrical input signal into the electrical output signal of the filter is effected in practice solely via the path of the acoustic wave, to be precise via the reflector(s). A direct signal transmission between an input transducer and an output transducer, that is to say circumventing the reflector structure(s), must in practice be precluded, even if said direct transmission between the input transducer and the output transducer occurs on an acoustic path. This is usually the case with resonator filters. In a resonator filter, the resonant property of the overall filter arrangement primarily determines the essential characteristic of such a filter, namely for the formation (bandwidth, group delay and the like) of its passband.
A relevant reflector filter whose essential characteristic of the passband is determined by the property of the reflector as such must be distinguished from a resonator filter here. Resonances of the entire filter arrangement must be avoided in a reflector filter.
A one-track reflector filter is known from IEEE Trains. Ultrasonics, Vol. 35 (1988), pages 61 ff, FIG. 3d. The reflector filter has an output transducer and an input transducer with transducer fingers positioned obliquely to the longitudinal axis of the filter, the oblique position of the one transducer being opposite to that of the other. The electrodes, oriented orthogonally to this axis, of the two reflectors reflect the acoustic waves propagating in the filter at an angle deviating from 180.degree.. The intention of this reflector filter with obliquely positioned transducer fingers is to minimize a direct acoustic signal transmission from the input transducer to the output transducer. The wave cancellation effect leading to this minimization is however dependent on the wavelength there, so that this result can only be achieved for a narrowband wavelength range. The oblique positioning of the transducer fingers provided also leads to additional losses. Such a reflector filter is difficult to calculate due to the obliquely oriented wave fronts occurring.